We have studied plasma formation and relaxation dynamics along with the corresponding topography modifications in fused silica and sapphire induced by single femtosecond laser pulses (800 nm and 120 fs). These materials, representative of high bandgap amorphous and crystalline dielectrics, respectively, require nonlinear mechanisms to absorb the laser light. The study employed a femtosecond time-resolved microscopy technique that allows obtaining reflectivity and transmission images of the material surface at well-defined temporal delays after the arrival of the pump pulse which excites the dielectric material. The transient evolution of the free-electron plasma formed can be followed by combining the time-resolved optical data with a Drude model to estimate transient electron densities and skin depths. The temporal evolution of the optical properties is very similar in both materials within the first few hundred picoseconds, including the formation of a high reflectivity ring at about 7 ps. In contrast, at longer delays (100 ps-20 ns) the behavior of both materials differs significantly, revealing a longer lasting ablation process in sapphire. Moreover, transient images of sapphire show a concentric ring pattern surrounding the ablation crater, which is not observed in fused silica. We attribute this phenomenon to optical diffraction at a transient elevation of the ejected molten material at the crater border. On the other hand, the final topography of the ablation crater is radically different for each material. While in fused silica a relatively smooth crater with two distinct regimes is observed, sapphire shows much steeper crater walls, surrounded by a weak depression along with cracks in the material surface. These differences are explained in terms of the most relevant thermal and mechanical properties of the material. Despite these differences the maximum crater depth is comparable in both material at the highest fluences used ͑16 J/cm 2 ͒. The evolution of the crater depth as a function of fluence can be described taking into account the individual bandgap of each material.

An unprecedented increase of kinetic energy of laser accelerated heavy ions is demonstrated. Ultra thin gold foils have been irradiated by an ultra short laser pulse at an intensity of 6 × 10 19 W/cm 2 . Highly charged gold ions with kinetic energies up to > 200 MeV and a bandwidth limited energy distribution have been reached by using 1.3 Joule laser energy on target. 1D and 2D Particle in Cell simulations show how a spatial dependence on the ions ionization leads to an enhancement of the accelerating electrical field. Our theoretical model considers a varying charge density along the target normal and is capable of explaining the energy boost of highly charged ions, leading to a higher efficiency in laser acceleration of heavy ions. PACS numbers:Laser driven ion acceleration has gained a wide scientific interest, as it is a promising ion source for investigation in basic plasma physics and for application in accelerator technology [1,2] related to bio-medical [3,4] and hadron research [5]. While the acceleration of protons and light ions are intensively investigated during the last decade, little is reported on acceleration of heavier ions [6]. Such knowledge is mandatory to achieve the objectives of upcoming new laser facilities [7,8], e.g. the exploration of nuclear, astrophysical questions as well as the potential use as beam lines for heavy ion radio therapy [9].Energies of heavy ions exceeding the mass number A 12 with E kin /u ∼ 1−2 MeV/u (energy per nucleon) have been reported so far [6,10], by using short pulse laser systems with laser pulse energies well above 20 J [11].In the following we report and discuss a considerable energy boost for acceleration of the highly charged heavy ions with only using 1.3 J on an ultra thin heavy material target. We accelerated ions up to E M ax /u > 1 MeV/u, with a bandwidth limited energy distribution. We found a remarkable deviation in the maximum energy to charge Z scaling in comparison to established models of Mora [12] and Schreiber [13,14].Presently used laser ion acceleration schemes like Target Normal Sheath Acceleration (TNSA) [15], or leaky light sail / Radiation Pressure Acceleration (RPA) [16][17][18], Coherent Acceleration of Ions by Laser (CAIL) [4,19], Break Out Afterburner (BOA) [20] make use of an energy transfer from laser to electrons and in a following step electrons accelerate the ions. In the typical physical picture, an ultra intense laser is focused on a thin target, ionizes it and displaces the electrons from the ion background by the laser field. This creates a high electrical field at the rear and front side of the target. The Coulomb attraction field of the ions circumvents the electrons escape and enables the acceleration of the ions. For ultra thin targets and relativistic laser intensities, the acceleration is enhanced by the transparency of the target and the relativistic kinematics of the electrons [18,[21][22][23]. Further optimization for the energies of light ions is proposed by a Coulomb exploding background of heavy ion constituen...

We have investigated the temporal and spatial evolution of the ablation process induced in fused silica upon irradiation with single 120 fs laser pulses at 800 nm. Time-resolved microscopy images of the surface reflectivity at 400 nm reveal the existence of a transient plasma distribution with annular shape surrounding the visible ablation crater. The material in this annular zone shows an increased reflectivity after irradiation, consistent with a local refractive index increase of approximately 0.01. White light interferometry measurements indicate a shallow surface depression in this outer region, most likely due to material densification.

In laser-based proton acceleration, nanostructured targets hold the promise to allow for significantly boosted proton energies due to strong increase of laser absorption. We used laser-induced periodic surface structures generated in-situ as a very fast and economic way to produce nanostructured targets capable of high-repetition rate applications. Both in experiment and theory, we investigate the impact of nanostructuring on the proton spectrum for different laser–plasma conditions. Our experimental data show that the nanostructures lead to a significant enhancement of absorption over the entire range of laser plasma conditions investigated. At conditions that do not allow for efficient laser absorption by plane targets, i.e. too steep plasma gradients, nanostructuring is found to significantly enhance the proton cutoff energy and conversion efficiency. In contrast, if the plasma gradient is optimized for laser absorption of the plane target, the nanostructure-induced absorption increase is not reflected in higher cutoff energies. Both, simulation and experiment point towards the energy transfer from the laser to the hot electrons as bottleneck.

Compact solid-state neutral particle analyzer in current mode Rev. Sci. Instrum. 83, 10D304 (2012) Source fabrication and lifetime for Li+ ion beams extracted from alumino-silicate sources Rev. Sci. Instrum. 83, 043303 (2012) Laser ion sources for radioactive beams (invited) Rev. Sci. Instrum. 83, 02A916 (2012) Producing persistent, high-current, high-duty-factor H− beams for routine 1 MW operation of Spallation Neutron Source (invited) Rev. Sci. Instrum. 83, 02A732 (2012) Additional information on Appl. Phys. Lett.